When it is required to obtain a cross sectional sample of a particular geological formation, it is known to use a core barrel assembly in place of a standard drill bit.
The core barrel assembly utilizes a specialized core bit attached to a number of outer barrels that are interconnected to make up the desired length. The core bit drills downwardly and has a central opening such that the core bit cuts around a column of the formation that is to be the sample. An inner barrel is provided within the outer barrel for receiving the core sample. The inner barrel is provided with an adaptor at the lower end that allows the core to pass into the inner barrel but not to fall back out.
The process of obtaining a core sample generally commences by connecting the core barrel assembly to the standard drill pipe string and lowering it to the bottom of the hole. Fluid is pumped through the drill string into the core barrel assembly where it passes through the inner barrel and the cavity between the inner and outer barrels to flush them of debris. A diverter ball is dropped through the drill string before commencement of sampling to seal the opening to the inner barrel so that fluid pumped down the drill string is passed only through the cavity between inner and outer barrels and coring commences. During coring, the core bit is designed to drill around a vertical column of the sample such that the inner barrel passes downwardly around the sample. A known problem that can occur in such a situation is that if the core column is not sufficiently stable, it can collapse downwardly within the inner barrel. The collapsed core column can create additional friction on the inner surface of the inner barrel resulting in jamming of the core.
Observations of the drilling fluid pressure, the torque and the rate of penetration can provide some indication of whether this core collapse has occurred, however it is not possible to rule out the possibility that changes in these values are the result of some other event (such as a change in the formation). The driller is therefore forced to make a decision that could result in continuing drilling when the core is jammed or stopping drilling when the core is not jammed, both situations resulting in an expensive loss of time and effort.
The present invention attempts to overcome at least in part the aforementioned problem of detecting collapse of a core sample within a core barrel assembly.